This invention relates to distortion linearizers, and more particularly to FET distortion phase and/or gain linearizers.
Satellite communications systems are finding increasing use for inter-continental and intra-continental data and entertainment information transfers. The basics of such systems are by now common knowledge, and include earth stations for transmitting information to, and for receiving repeated or translated information from, a satellite, which is often geosynchronous. The high capital costs of communications satellites requires that the greatest possible use be made of their facilities, to lower the per-unit cost of communications. The severe operating environment in space, coupled with the inability to access the orbiting satellite to effect repairs, places stringent requirements on the systems and components of a satellite and its communication system payload. A plurality of communications channels are ordinarily provided, with enhanced channel-to-channel isolation by use of combined frequency and polarization diversity, as described in U.S. patent application Ser. No. 07/772,207, filed Oct. 7, 1991 in the name of Wolkstein. The information signal at the satellite must be amplified before retransmission back to earth for reception by an earth station. In principle, the information signals received at the satellite could be amplified by a single power amplifier. However, because of the linearity limitations of amplifiers which are available at the current state of the art for operation at the desired signal amplitude or power output levels, excessive intermodulation distortion occurs when multiple channels are processed in a single amplifier.
In order to maximize of the life of the satellite, the weight of every additional component must be assessed against the reduction in the useful lifetime of the satellite as a result of the reduced payload of expendable propellant (fuel or fuel-plus-oxidizer) which the component displaces. Thus, the tradeoffs of communications system performance versus weight are carefully evaluated. This evaluation is rendered complex because of other factors which must be considered, such as power consumption, reliability and payload performance.
As described by the aforementioned Wolkstein application, conventional communications systems use individual power amplifiers in each channel to reduce intermodulation distortion. However, some forms of distortion, such as phase distortion dependent upon instantaneous signal amplitude, and signal compression with increasing signal level, are not ameliorated by the single-channel amplifying technique. In order to maximize the output power from the power amplifier in each channel, the signal level is increased to a level at which significant distortion occurs, and a distortion corrector (often termed a pre- or post-equalizer) is added in the amplifier signal path.
Many different types of equipments termed balancers, compensators and equalizers are used in systems generally and in communications systems in particular. Thus, "balancers" may be weights used on rotating devices which reduce physical vibration, or may include a potentiometer connected to the electrodes of the tubes of a push-pull vacuum-tube amplifier to reduce harmonic distortion by making the tube's operating transfer functions as similar as possible. A "compensator" may be gas diverters affixed to the barrel of a small arm or cannon to reduce recoil, a magnet attached to compass, or it may be an electrical device, of which one example is a linear (nominally independent of amplitude) variable amplitude-versus-frequency (slope) device for maintaining constant gain in transmission-line systems such as cable television systems (CATV) in the face of different coaxial cable lengths or, when thermally controlled, in the face of performance variations caused by daily and seasonal temperature variations. Similarly, an "equalizer" may be a mechanical apparatus for distributing a load across several supports or an electric conductor joining various equipotential locations in a circuit. Also, "equalizer" is another term for a variable or thermal cable compensator, as described above. A bridged tee slope equalizer including complex reactive bridge networks is described in U.S. Pat. No. 4,967,169, issued Oct. 30, 1990 to Sun et al. A distortion equalizer corrects, in some way, for the distortion of an associated non-linear circuit. A "predistortion" equalizer is a non-linear apparatus, inserted in the signal path between a signal source and a non-linear apparatus such as a signal amplifier, for predistorting the signal in response to amplitude so that the amplitude andor phase distortion introduced by the following non-linear stage is wholly or partially cancelled. A post-distortion equalizer performs the same function at the output of the non-linear stage. Since the non-linear device for which compensation is required is usually a power amplifier, a post-distortion equalizer must handle higher signal amplitudes than a predistortion equalizer, for which reason predistortion equalizers are preferred.
As mentioned, satellite system tradeoffs are carefully evaluated in terms of power consumption, reliability, weight and performance. A great deal of attention has been directed to the tradeoffs between solid-state amplifiers and travelling-wave tubes as channel amplifiers for satellite communications, and at this time both are being improved and both types are currently used for frequency ranges from about 2 Gigahertz (GHz) to 13 GHz. Distortion equalizers include transmission schemes such as the dual-gate, common-source FET scheme described in U.S. Pat. No. 4,465,980, issued Aug. 14, 1984 to Huang et al, in which the signal is applied to one gate, and detected signal from a signal sample is applied to the other gate to generate the desired distortion. The signal sample is produced by a directional coupler. Directional couplers appear in other predistortion circuits, such as U.S. Pat. No. 4,109,212, issued Aug. 22, 1971 in the name of Donnell et al; U.S. Pat. No. 4,283,684, issued Aug. 11, 1981 in the name of Satoh; U.S. Pat. No. 4,564,816, issued Jan. 14, 1986 in the name of Kumar et al; and U.S. Pat. No. 4,588,958, issued May 13, 1986, in the name of Katz et al. These prior distortion equalizers have a salient disadvantage for satellite use, in that they use directional couplers. Such directional couplers are often designed as waveguide branch devices, which are assemblages of two parallel "through" waveguides, with a plurality of "branch" waveguides extending therebetween, which are dimensioned to produce the desired linear power division and linear phase shift. Such waveguide devices must have dimensions which are significant portions of a wavelength at the frequency of operation, and so they cannot be miniaturized. As a result, the waveguide directional coupler for a satellite predistortion equalizer, and possibly other components of some equalizers, tend to make each distortion equalizer bulky and heavy. This is particularly disadvantageous in multi-channel systems, because each channel includes a predistortion equalizer.
Reliability of satellite systems is enhanced by redundancy schemes. In many satellite communication systems, switched routing schemes allow high-priority signals to be routed to operative channels in the event of a channel failure. Among the system portions which are more likely to fail are the amplifiers. Consequently, redundancy schemes often involve switching the amplifiers among channels, together with supernumerary, ordinarily unused amplifiers, which can be switched into a channel to replace a failed amplifier. A concomitant of such a redundancy scheme is that each amplifier, and its associated distortion equalizer, must be capable of broadband frequency performance.
U.S. Pat. No. 5,038,113, issued Aug. 6, 1991 in the name of Katz et al, describes a transmission-type predistortion equalizer comprising an FET with its source-to-drain conductive path coupled in series with a transmission line, in contrast to the above-mentioned Huang et al common-source arrangement, and with a gate-to-ground reactance and gate bias selected to produce the desired level of distortion of the signals. In general, this transmission-type FET acts as a lossy transmission element, in which the loss decreases with increasing signal, to produce signal expansion. The signal expansion with increasing signal level offsets the signal compression occasioned by the associated amplifier. The phase shift through the transmission FET is also affected by the signal level. Several "modes" of operation of the Katz et al transmission FET distortion equalizer have been identified, which depend upon the gate bias voltage and the gate impedance. Three modes, each with about 5% bandwidth, have been identified, with both increasing and decreasing phase shift as a function of increasing signal level. A fourth mode provides gain expansion at frequencies below about 3 GHz with the FETs currently available. A fifth mode of operation is relatively broadband, and provides useful gain expansion at and above Ku-band (about 12 GHz). While this fifth mode of operation provides gain expansion in a region which is of interest at certain satellite communication frequencies, it provides phase shift which decreases (less time delay) with increasing signal power level, which may not be suitable for equalizing those amplifiers which have a similar phase distortion. To equalize distortion for those amplifiers subject to signal or gain compression together with decreasing phase shift in response to increasing signal power level, the distortion equalizer must have gain expansion coupled with phase shift which increases (more time delay) with increasing power level.
A copending patent application entitled, "Balanced Reflective Nonlinear Processor using FET," Ser. No. 07/753,164, filed Aug. 30, 1991 in the name of Katz et al, describes a reflective, balanced arrangement of transmission FET distortion generator operated in a reflective mode, which provides gain expansion together with increasing phase shift, as a function of increasing signal power level. This arrangement uses a coupler as part of the reflective balanced arrangements.
An improved distortion generator is desired.